The parent patent application, U.S. Ser. No. 07/598,476, describes a tunable liquid-crystal etalon filter comprising a liquid crystal filling a Fabry-Perot cavity defined on each end by dielectric interference mirrors. Electrodes are affixed to the outer surfaces of the interference mirrors. When voltage is applied across the electrodes, the effective dielectric constant of the liquid crystal is changed, thereby changing the optical length of the Fabry-Perot cavity and also changing the resonant frequency of the cavity. Accordingly, the filter selectively resonates and passes an optical frequency according to the applied electric field. Saunders had previously disclosed in U.S. Pat. No. 4,779,959 a liquid-crystal light modulator that uses metallic mirrors rather than interference mirrors. Because of the mirror losses, the transmission is relatively low and the resonances are relatively wide so that it can function not as a filter but only as a modulator for a well defined wavelength.
Many applications, particularly in a fiber-optic communication network, require that an optical filter be polarization-independent, that is, that the spectral characteristics of the filter be independent of the polarization of light being filtered. The grandparent application Ser. No. 07/577,220 discloses that polarization independence can be achieved if the liquid crystal is aligned to be parallel at each of the electrodes but is twisted an odd multiple of 90.degree. between the electrodes. Although this approach has been demonstrated, it suffers from a relatively small spectral range over which it can be electrically tuned.